1. Field of the Invention
The present invention relates to a protection circuit for preventing transistor breakdown during overvoltage output operation in a power amplifier having a GaAs heterojunction bipolar transistor (HBT) a Si bipolar transistor.
2. Description of Related Art
Monolithic microwave integrated circuits (MMIC) and modules (hybrid ICs and multichip modules) using GaAs metal-semiconductor field effect transistors (MESFET), GaAs high electron mobility transistors (HEMT), or GaAs-based HBTs are widely used in power amplifiers for mobile communications devices. Of these, GaAs-based HBTs offer the following three advantages over conventional FETs, and are therefore considered promising as power module elements for future mobile communication devices. That is, a GaAs-based HBT:
1) does not require a negative gate bias voltage, and can therefore be used for simple power supply operations; PA1 2) can be used for on/off operations without an analog switch on the drain side, similarly to a Si-MOSFET; PA1 3) has a high output power density, enabling a particular rated output power to be achieved from a device that is smaller than a FET power amplifier with comparable output power.
HBT power amplifiers using these particular properties of HBTs are starting to be used in 2 W to 4 W high output mobile telephones conforming to Europe's GSM (Global System for Mobile Communications) standard, a 900 MHz system that is the most widely used mobile telephone service in Europe.
FIG. 10 is a typical circuit diagram for an HBT power amplifier 500 used in a GSM mobile telephone. Referring to FIG. 10, input terminal 501 is the input terminal for RF signals to be power amplified. Output terminal 502 is the output terminal for amplified signals. Transistors 503 to 505 are heterojunction bipolar transistors for signal amplifying. Transistors 506 to 511 are bias HBTs. Terminals 512 to 514 are terminals for applying collector bias voltages Vcl to Vc3. Terminal 515 is the supply terminal for the power supply voltage Vcc. Terminal 516 is the supply terminal for power control voltage Vpc for controlling the base voltage of amplifying transistors 503 to 505 using bias transistors 506 to 511. As shown in the figure, this HBT power amplifier 500 also comprises resistors 517 to 542, capacitors 543 to 552, and microwave lines 553 to 557.
A typical HBT power amplifier has a high low-frequency gain and is more susceptible to low frequency oscillation than an FET power amplifier. To prevent this low frequency oscillation, HBT power amplifier 500 has an RC feedback circuit comprising resistor 521 and capacitor 544 disposed between the collector and base of transistor 503; an RC feedback circuit comprising resistor 525 and capacitor 545 disposed between the collector and base of transistor 504; and an RC feedback circuit comprising resistor 529 and capacitor 548 disposed between the collector and base of transistor 505.
A current of approximately 2 A flows to circuits in a mobile telephone based on the GSM standard. A means such as a regulator or cut-off circuit that operates when an overvoltage is applied could be provided between the power terminal and battery as a means of protecting transistors in the HBT power amplifier 500. However, such cut-off circuits and regulators are typically large-scale, high power consumption devices. It is therefore not possible to dispose a cut-off circuit or regulator between the battery and supply terminal of the HBT power amplifier 500, and the supply terminal is directly connected to an internal battery.
It is also desirable to provide an isolator between the output terminal of the HBT power amplifier 500 and a downstream circuit (such as an antenna). This isolator is used for suppressing variations in the load curve of HBT power amplifier 500 when the load impedance of a downstream circuit varies. This isolator, however, is typically comparable to the chip size of the HBT power amplifier 500, and is therefore omitted in GSM standard mobile telephones because of the high demand for smaller mobile telephones.
If the supply voltage rises to a particular level above the recommended operating conditions (3 V to 3.6 V) during battery recharging, for example, rises to 4.5 V to 5.5 V, and the load impedance of downstream circuits varies greatly from the normal 50 .OMEGA. level, the load characteristic curve of the last transistor 505 in a HBT power amplifier 500 for GSM devices described as noted above may fluctuate excessively, causing the peak collector voltage Vce to exceed the breakdown voltage and transistor 505 to fail.
The last transistor 505 can fail as a result of load fluctuations during battery charging as described below, but failure can also result from an overvoltage being applied to the collector of the last transistor 505.
It should also be noted that a feedback circuit 600 having a diode 604 as shown in FIG. 11 has also been used to resolve the above-noted problems relating to an overvoltage supply in applications other than mobile telephones and other mobile devices, particularly in the field of optical communications. The feedback circuit 600 shown in FIG. 11 is used primarily in the signal amplifier 601 disposed in the front end of an optical communications receiver. When an overvoltage supply is input the diode 604 becomes conductive, thereby reducing the power applied to the signal input terminal 602 of the signal amplifier 601 to prevent signal amplifier 601 failure, wave distortion, and overload output.
High output power amplifiers producing 1 W or more are used in GSM standard mobile telephones. Providing a feedback circuit 600 as described above to the input/output terminals of such a high output power amplifier, however, cannot effectively protect the internal transistors of the power amplifier, and in particular cannot protect the last transistor 505. This is because failure and thermal damage to transistors inside the amplifier is due not primarily to an overvoltage supply being input to the amplifier, but is due to the peak collector voltage Vce exceeding the breakdown voltage due to load fluctuations during overloaded operation of transistors in the amplifier.
The load fluctuation characteristic during overload operation of the transistors when a tuner 560 produces a variable load impedance at the output terminal 502 of the HBT power amplifier 500 is considered next below. FIG. 12 shows the second and third stage circuits of a HBT power amplifier 500 to which this tuner 560 is connected.
FIG. 13 shows the load curve and Ic-Vce curve of the last transistor 505 when tuner 560 is connected. Point Al on the graph indicates the normal collector bias voltage V.sub.c3-1 (typically 3.2 V) applied to terminal 514 when the base current is I.sub.b2. Curve cl is the load curve when the load impedance of the tuner 560 is normal (that is, 50 .OMEGA.) and the voltage standing wave ratio (VSWR) is 1:1. Curve c2 is the load curve when there is a mismatch, that is, the load impedance deviates from the normal impedance (50 .OMEGA. in this case) and the VSWR is 8:1 to 10:1. Such a mismatch occurs when, for example, a mobile telephone comprising this HBT power amplifier 500 passes a highly conductive object, such as a steel utility pole.
As will be known from comparing curves c1 and c2, if the standing wave ratio of the output terminal is increased and the output mismatch is increased, load curve fluctuation will increase and the peak collector voltage Vce will approach the transistor breakdown voltage area 1 indicated by a circle line in the figure.
FIG. 14 shows the load curve and Ic-Vce curve of the last transistor 505 when the collector bias voltage applied to terminal 514 is a voltage V.sub.c3-2 (5.0 V in this example) higher than V.sub.c3-1 (3.2 V as noted above). Point A2 on the graph indicates the collector bias voltage V.sub.c3-2 applied to terminal 514 when the base current is I.sub.b2. As is curve c1 in FIG. 13, curve c3 is the load curve when the load impedance of the tuner 560 is normal (that is, 50 .OMEGA.) and the voltage standing wave ratio (VSWR) is 1:1. As is curve c2 in FIG. 13, curve c4 is the load curve when there is a mismatch, that is, the load impedance deviates from the normal impedance (50 .OMEGA. in this case) and the VSWR is 8:1 to 10:1. As will be known from curve c2 in FIG. 13 and curve c4 in FIG. 14, the peak collector voltage Vce exceeds the breakdown voltage (enters breakdown area 1 in the figures) and increases the potential for transistor breakdown when the collector bias voltage exceeds a particular level.
When current gain is improved and parasitic resistance and capacitance are reduced to improve transistor characteristics during low voltage operation, that is, operation at the standard operating voltage of 3 V to 3.6 V in mobile telephones designed for low voltage operation, the breakdown voltage also tends to drop. Due to the potential for supply voltage variations and load variations, it is therefore desirable to provide an isolator for suppressing variations in the load curve due to fluctuations in the load impedance of downstream circuits. In systems such as GSM standard mobile telephones in which such an isolator is not provided because of the demand for compact size, it is therefore important to prevent transistor breakdown caused by overload output of the collector voltage Vce resulting from load fluctuations during overload operation of amplifier transistors.